Dissociation of the effects of amylin on osteoblast proliferation and bone resorption

Understanding the structural dynamics of human islet amyloid polypeptide: Advancements in and applications of ion-mobility mass spectrometry

Human islet amyloid polypeptide (hIAPP) forms amyloid deposits that contribute to β-cell death in pancreatic islets and are considered a hallmark of Type II diabetes Mellitus (T2DM). Evidence suggests that the early oligomers of hIAPP formed during the aggregation process are the primary pathological agent in islet amyloid induced β-cell death. The self-assembly mechanism of hIAPP, however, remains elusive, largely due to limitations in conventional biophysical techniques for probing the distribution or capturing detailed structures of the early, structurally dynamic oligomers. The advent of Ion-mobility Mass Spectrometry (IM-MS) has enabled the characterisation of hIAPP early oligomers in the gas phase, paving the way towards a deeper understanding of the oligomerisation mechanism and the correlation of structural information with the cytotoxicity of the oligomers. The sensitivity and the rapid structural characterisation provided by IM-MS also show promise in screening hIAPP inhibitors, categorising their modes of inhibition through "spectral fingerprints". This review delves into the application of IM-MS to the dissection of the complex steps of hIAPP oligomerisation, examining the inhibitory influence of metal ions, and exploring the characterisation of hetero-oligomerisation with different hIAPP variants. We highlight the potential of IM-MS as a tool for the high-throughput screening of hIAPP inhibitors, and for providing insights into their modes of action. Finally, we discuss advances afforded by recent advancements in tandem IM-MS and the combination of gas phase spectroscopy with IM-MS, which promise to deliver a more sensitive and higher-resolution structural portrait of hIAPP oligomers. Such information may help facilitate a new era of targeted therapeutic strategies for islet amyloidosis in T2DM.

Accelerated Development With Increased Bone Mass and Skeletal Response to Loading Suggest Receptor Activity Modifying Protein-3 as a Bone Anabolic Target

Knockout technologies provide insights into physiological roles of genes. Studies initiated into endocrinology of heteromeric G protein-coupled receptors included deletion of receptor activity modifying protein-3, an accessory protein that alters ligand selectivity of calcitonin and calcitonin-like receptors. Initially, deletion of Ramp3^-/- appeared phenotypically silent, but it has emerged that mice have a high bone mass phenotype, and more subtle alterations to angiogenesis, amylin homeostasis, and a small proportion of the effects of adrenomedullin on cardiovascular and lymphatic systems. Here we explore in detail, effects of Ramp3^-/- deletion on skeletal growth/development, bone mass and response of bone to mechanical loading mimicking exercise. Mouse pups lacking RAMP3 are healthy and viable, having accelerated development of the skeleton as assessed by degree of mineralisation of specific bones, and by microCT measurements. Specifically, we observed that neonates and young mice have increased bone volume and mineralisation in hindlimbs and vertebrae and increased thickness of bone trabeculae. These changes are associated with increased osteoblast numbers and bone apposition rate in Ramp3^-/- mice, and increased cell proliferation in epiphyseal growth plates. Effects persist for some weeks after birth, but differences in gross bone mass between RAMP3 and WT mice lose significance in older animals although architectural differences persist. Responses of bones of 17-week old mice to mechanical loading that mimics effects of vigorous exercise is increased significantly in Ramp3^-/- mice by 30% compared with WT control mice. Studies on cultured osteoblasts from Ramp3^-/- mice indicate interactions between mRNA expression of RAMPs1 and 3, but not RAMP2 and 3. Our preliminary data shows that Ramp3^-/- osteoblasts had increased expression β-catenin, a component of the canonical Wnt signalling pathway known to regulate skeletal homeostasis and mechanosensitivity. Given interactions of RAMPs with both calcitonin and calcitonin-like receptors to alter ligand selectivity, and with other GPCRs to change trafficking or ligand bias, it is not clear whether the bone phenotype of Ramp3^-/- mice is due to alterations in signalling mediated by one or more GPCRS. However, as antagonists of RAMP-interacting receptors are growing in availability, there appears the likelihood that manipulation of the RAMP3 signalling system could provide anabolic effects therapeutically.

Calcitonin and Bone Physiology: In Vitro, In Vivo, and Clinical Investigations

Calcitonin was discovered as a peptide hormone that was known to reduce the calcium levels in the systemic circulation. This hypocalcemic effect is produced due to multiple reasons such as inhibition of bone resorption or suppression of calcium release from the bone. Thus, calcitonin was said as a primary regulator of the bone resorption process. This is the reason why calcitonin has been used widely in clinics for the treatment of bone disorders such as osteoporosis, hypercalcemia, and Paget's disease. However, presently calcitonin usage is declined due to the development of efficacious formulations of new drugs. Calcitonin gene-related peptides and several other peptides such as intermedin, amylin, and adrenomedullin (ADM) are categorized in calcitonin family. These peptides are known for the structural similarity with calcitonin. Aside from having a similar structure, these peptides have few overlapping biological activities and signal transduction action through related receptors. However, several other activities are also present that are peptide specific. In vitro and in vivo studies documented the posttreatment effects of calcitonin peptides, i.e., positive effect on bone osteoblasts and their formation and negative effect on osteoclasts and their resorption. The recent research studies carried out on genetically modified mice showed the inhibition of osteoclast activity by amylin, while astonishingly calcitonin plays its role by suppressing osteoblast and bone turnover. This article describes the review of the bone, the activity of the calcitonin family of peptides, and the link between them.

The Activity of Peptides of the Calcitonin Family in Bone

Calcitonin was discovered over 50 yr ago as a new hormone that rapidly lowers circulating calcium levels. This effect is caused by the inhibition of calcium efflux from bone, as calcitonin is a potent inhibitor of bone resorption. Calcitonin has been in clinical use for conditions of accelerated bone turnover, including Paget's disease and osteoporosis; although in recent years, with the development of drugs that are more potent inhibitors of bone resorption, its use has declined. A number of peptides that are structurally similar to calcitonin form the calcitonin family, which currently includes calcitonin gene-related peptides (αCGRP and βCGRP), amylin, adrenomedullin, and intermedin. Apart from being structurally similar, the peptides signal through related receptors and have some overlapping biological activities, although other activities are peptide specific. In bone, in vitro studies and administration of the peptides to animals generally found inhibitory effects on osteoclasts and bone resorption and positive effects on osteoblasts and bone formation. Surprisingly, studies in genetically modified mice have demonstrated that the physiological role of calcitonin appears to be the inhibition of osteoblast activity and bone turnover, whereas amylin inhibits osteoclast activity. The review article focuses on the activities of peptides of the calcitonin family in bone and the challenges in understanding the relationship between the pharmacological effects and the physiological roles of these peptides.

Human Islet Amyloid Polypeptide N-Terminus Fragment Self-Assembly: Effect of Conserved Disulfide Bond on Aggregation Propensity

Amyloid formation by human islet amyloid polypeptide (hIAPP) has long been implicated in the pathogeny of type 2 diabetes mellitus (T2DM) and failure of islet transplants, but the mechanism of IAPP self-assembly is still unclear. Numerous fragments of hIAPP are capable of self-association into oligomeric aggregates, both amyloid and non-amyloid in structure. The N-terminal region of IAPP contains a conserved disulfide bond between cysteines at position 2 and 7, which is important to hIAPP's in vivo function and may play a role in in vitro aggregation. The importance of the disulfide bond in this region was probed using a combination of ion mobility-based mass spectrometry experiments, molecular dynamics simulations, and high-resolution atomic force microscopy imaging on the wildtype 1-8 hIAPP fragment, a reduced fragment with no disulfide bond, and a fragment with both cysteines at positions 2 and 7 mutated to serine. The results indicate the wildtype fragment aggregates by a different pathway than either comparison peptide and that the intact disulfide bond may be protective against aggregation due to a reduction of inter-peptide hydrogen bonding. Graphical Abstract ᅟ.

Amylin structure-function relationships and receptor pharmacology: implications for amylin mimetic drug development

Amylin is an important, but poorly understood, 37 amino acid glucoregulatory hormone with great potential to target metabolic diseases. A working example that the amylin system is one worth developing is the FDA-approved drug used in insulin-requiring diabetic patients, pramlintide. However, certain characteristics of pramlintide pharmacokinetics and formulation leave considerable room for further development of amylin-mimetic compounds. Given that amylin-mimetic drug design and development is an active area of research, surprisingly little is known about the structure/function relationships of amylin. This is largely due to the unfavourable aggregative and solubility properties of the native peptide sequence, which are further complicated by the composition of amylin receptors. These are complexes of the calcitonin receptor with receptor activity-modifying proteins. This review explores what is known of the structure-function relationships of amylin and provides insights that can be drawn from the closely related peptide, CGRP. We also describe how this information is aiding the development of more potent and stable amylin mimetics, including peptide hybrids.

Amylin exerts osteogenic actions with different efficacy depending on the diabetic status

Amylin displays osteogenic features, but its role in diabetic osteopenia is unclear. We examined the possible osteogenic action of amylin infusion for 3days into fructose-induced insulin-resistant (IR) and streptozotocin-induced type 2 diabetic (T2D) and normal (N) rats. Amylin failed to affect glycaemia or parathyroid hormone levels in any group, but reduced hyperinsulinemia in IR rats. In N rats, amylin increased bone formation rate and reduced osteoclast surface and erosive surface in the femoral metaphysis, and increased osteoprotegerin (OPG)/receptor activator of NFκB ligand (RANKL) mRNA ratio in the tibia. In T2D rats, amylin normalized trabecular structure parameters and increased osteoblast number and osteocalcin (OC) expression in long bones. In contrast, in IR rats, no apparent osteogenic effect of amylin in the femur was observed, although both OC and OPG/RANKL ratio were increased in the tibia. Our findings demonstrate a different osteogenic efficacy of amylin in two diabetic settings.

Energy Balance, Myostatin, and GILZ: Factors Regulating Adipocyte Differentiation in Belly and Bone

Peroxisome proliferator-activated receptor gamma (PPAR-gamma) belongs to the nuclear hormone receptor subfamily of transcription factors. PPARs are expressed in key target tissues such as liver, fat, and muscle and thus they play a major role in the regulation of energy balance. Because of PPAR-gamma's role in energy balance, signals originating from the gut (e.g., GIP), fat (e.g., leptin), muscle (e.g., myostatin), or bone (e.g., GILZ) can in turn modulate PPAR expression and/or function. Of the two PPAR-gamma isoforms, PPAR-gamma2 is the key regulator of adipogenesis and also plays a role in bone development. Activation of this receptor favors adipocyte differentiation of mesenchymal stem cells, while inhibition of PPAR-gamma2 expression shifts the commitment towards the osteoblastogenic pathway. Clinically, activation of this receptor by antidiabetic agents of the thiazolidinedione class results in lower bone mass and increased fracture rates. We propose that inhibition of PPAR-gamma2 expression in mesenchymal stem cells by use of some of the hormones/factors mentioned above may be a useful therapeutic strategy to favor bone formation.

Reduced amylin levels are associated with low bone mineral density in women with anorexia nervosa

CONTEXT

Anorexia nervosa, characterized by extreme low body weight due to reduced nutrient intake, is associated with severe bone loss. Peptide hormones, including amylin, GIP, and GLP2, are released immediately after nutrient intake and may be involved in the regulation of bone turnover.

OBJECTIVE

To investigate fasting levels of amylin, GIP, and GLP2 and their relationships with bone mineral density (BMD) in women with anorexia nervosa compared to healthy controls.

DESIGN

Cross-sectional.

SETTING

Clinical Research Center.

STUDY PARTICIPANTS

15 women with anorexia nervosa and 16 healthy controls.

INTERVENTION

None.

MAIN OUTCOME MEASURES

Fasting serum amylin, GIP, and GLP2, and BMD.

RESULTS

Women with anorexia nervosa had significantly lower fasting serum amylin and GIP levels than healthy controls. Fasting serum GLP2 levels were not significantly different between groups. Fasting amylin levels were positively associated with BMD and Z-score at the PA spine, total hip, and femoral neck. Fasting amylin levels were also positively associated with weight and percent fat; after controlling for these variables, amylin was still a significant predictor of BMD and Z-score at the femoral neck and of Z-score at the total hip. In the anorexia nervosa group, there was a trend toward an inverse association between amylin and C-terminal telopeptide (CTX) levels (R=-0.47, p=0.08). GIP and GLP2 levels did not predict BMD at any site.

CONCLUSION

Decreased secretion of amylin may be a mechanism through which reduced nutrient intake adversely affects BMD in anorexia nervosa.

Amylin(1-8) is devoid of anabolic activity in bone

Amylin(1-8), a cyclic peptide consisting of the eight N-terminal amino acids of the 37-amino acid peptide amylin, has been shown to induce proliferation of primary osteoblasts and to induce bone formation in healthy male mice, whereas no data on efficacy in bone disease-related models have been reported. Therefore, we evaluated any effects of amylin(1-8) in ovariectomized rats with established osteopenia, a model for postmenopausal osteoporosis. At doses up to 100 nmol/kg/day, a dose highly effective in healthy mice, amylin(1-8) was unable to increase bone mineral density in ovariectomized rats during an 8-week treatment period. Histomorphometric analysis of the tibia indicated that amylin(1-8) did not change bone histomorphometric parameters. In an attempt to verify any potential biological effects of amylin(1-8), we investigated the efficacy of this peptide in various in vitro assays. Experiments designed to confirm previously published results on the proliferative effects of amylin(1-8) on primary osteoblasts failed to show any response. Amylin(1-8) was able to partially displace (125)I-rat amylin(1-37) from amylin receptors composed of the calcitonin receptor and RAMP1, indicating specific interaction of the peptide with the amylin binding site. However, in vitro efficacy assays with amylin(1-8) in calcitonin receptor-RAMP-positive HEK293T and MCF7 cells failed to reveal any agonist activity of amylin(1-8), whereas amylin(1-37) showed the expected agonist activity. In conclusion, our results indicate that amylin(1-8) does not show agonist activity on amylin receptors, does not affect osteoblast proliferation, and is devoid of anabolic activity in bone.

Helix stabilization precedes aqueous and bilayer-catalyzed fiber formation in islet amyloid polypeptide

Islet amyloid polypeptide (IAPP) is an unstructured polypeptide hormone that is cosecreted with insulin. In patients with type 2 diabetes, IAPP undergoes a transition from its natively disordered state to a highly ordered, all-beta-strand amyloid fiber. Although predominantly disordered, IAPP transiently samples alpha-helical structure in solution. IAPP adopts a fully helical structure when bound to membrane surfaces in a process associated with catalysis of amyloid formation. Here, we use spectroscopic techniques to study the structure of full-length, monomeric IAPP under amyloidogenic conditions. We observe that the residues with helical propensity in solution (1-22) also form the membrane-associated helix. Additionally, reduction of the N-terminal disulfide bond (Cys2-Cys7) decreases the extent of helix formed throughout this region. Through manipulation of sample conditions to increase or decrease the amount of helix, we show that the degree of helix formed affects the rate of amyloid assembly. Formation of helical structure is directly correlated with enhanced amyloid formation both on the membrane surface and in solution. These observations support suggested mechanisms in which parallel helix associations bring together regions of the peptide that could nucleate beta-strand structure. Remarkably, stabilization of non-amyloid structure appears to be a key intermediate in assembly of IAPP amyloid.

Actions of fibroblast growth factor-8 in bone cells in vitro

The fibroblast growth factors (FGFs) are a group of at least 25 structurally related peptides that are involved in many biological processes. Some FGFs are active in bone, including FGF-1, FGF-2, and FGF-18, and recent evidence indicates that FGF-8 is osteogenic, particularly in mesenchymal stem cells. In the current study, we found that FGF-8 was expressed in rat primary osteoblasts and in osteoblastic UMR-106 and MC3T3-E1 cells. Both FGF-8a and FGF-8b potently stimulated the proliferation of osteoblastic cells, whereas they inhibited the formation of mineralized bone nodules in long-term cultures of osteoblasts and reduced the levels of osteoblast differentiation markers, osteocalcin, and bone sialoprotein. FGF-8a induced the phosphorylation of p42/p44 mitogen-activated protein kinase (MAPK) in osteoblastic cells; however, its mitogenic actions were not blocked by either the MAPK kinase (MEK) inhibitor U-0126 or the PI 3-kinase (PI3K) inhibitor LY-294002. Interestingly, FGF-8a, unlike FGF-8b and other members of the family, inhibited osteoclastogenesis in mouse bone marrow cultures, and this was via a receptor activator of NF-kappaB ligand (RANKL)/osteoprotegerin (OPG)-independent manner. However, FGF-8a did not affect osteoclastogenesis in RAW 264.7 cells (a macrophage cell line devoid of stromal cells) exogenously stimulated by RANKL, nor did it affect mature osteoclast function as assessed in rat calvarial organ cultures and isolated mature osteoclasts. In summary, we have demonstrated that FGF-8 is active in bone cells, stimulating osteoblast proliferation in a MAPK-independent pathway and inhibiting osteoclastogenesis via a RANKL/OPG-independent mechanism. These data suggest that FGF-8 may have a physiological role in bone acting in an autocrine/paracrine manner.

Preptin, another peptide product of the pancreatic beta-cell, is osteogenic in vitro and in vivo

Several hormones that regulate nutritional status also impact on bone metabolism. Preptin is a recently isolated 34-amino acid peptide hormone that is cosecreted with insulin and amylin from the pancreatic beta-cells. Preptin corresponds to Asp(69)-Leu(102) of pro-IGF-II. Increased circulating levels of a pro-IGF-II peptide complexed with IGF-binding protein-2 have been implicated in the high bone mass phenotype observed in patients with chronic hepatitis C infection. We have assessed preptin's activities on bone. Preptin dose-dependently stimulated the proliferation (cell number and DNA synthesis) of primary fetal rat osteoblasts and osteoblast-like cell lines at periphysiological concentrations (>10(-11) M). In addition, thymidine incorporation was stimulated in murine neonatal calvarial organ culture, likely reflecting the proliferation of cells from the osteoblast lineage. Preptin did not affect bone resorption in this model. Preptin induced phosphorylation of p42/p44 MAP kinases in osteoblastic cells in a dose-dependent manner (10(-8)-10(-10) M), and its proliferative effects on primary osteoblasts were blocked by MAP kinase kinase inhibitors. Preptin also reduced osteoblast apoptosis induced by serum deprivation, reducing the number of apoptotic cells by >20%. In vivo administration of preptin increased bone area and mineralizing surface in adult mice. These data demonstrate that preptin, which is cosecreted from the pancreatic beta-cell with amylin and insulin, is anabolic to bone and may contribute to the preservation of bone mass observed in hyperinsulinemic states such as obesity.

Amylin fasting plasma levels are decreased in patients with osteoporosis

Amylin is a polypeptide hormone produced in pancreatic beta-cells that belongs to the family of calcitonin gene-related peptides. There is a 20% sequence homology between amylin and calcitonin and 44% homology with calcitonin gene-related peptide. Amylin and its fragments stimulate the proliferation of osteoblasts, inhibit bone resorption, and increase bone density and the amount of bone mass. We measured amylin total and unreduced amylin fasting plasma levels in patients with osteoporosis ( n=28; 3 men, 25 women; mean age 65 years), type 2 diabetes mellitus ( n=10; 5 men, 5 women; 64 years), and in the control group ( n=24; 11 men, 13 women; 53 years) using an ELISA kit with immunofluorescent detection (Linco). Amylin total plasma levels in patients with osteoporosis were 3.33+/-0.46 pmol/l (mean+/-SEM), in patients with type 2 diabetes 6.29+/-1.47 pmol/l (mean+/-SEM), and in the control group 8.48+/-3.12 pmol/l (mean+/-SEM). Mean plasma levels were lower in patients with osteoporosis than in patients with type 2 diabetes and in the control group. Unreduced amylin plasma levels in patients with osteoporosis ( n=28) were 2.51+/-0.87 pmol/l (mean+/-SEM), in patients with type 2 diabetes ( n=10) 4.15+/-0.95 pmol/l (mean+/-SEM) and in the control group ( n=5) 13.50+/-3.94 pmol/l (mean+/-SEM). Plasma levels were significantly lower in patients with osteoporosis than in patients with type 2 diabetes ( P<0.01) and in the control group ( P<0.001). Amylin plasma levels are decreased in patients with osteoporosis. Amylin deficiency in these patients may contribute to the development of osteoporosis. Amylin should be investigated in relation to the pharmacological treatment of osteoporosis.

Systemic administration of a novel octapeptide, amylin-(1---8), increases bone volume in male mice

Amylin increases bone mass when administered systemically to mice. However, because of its size, the full peptide is not an ideal candidate for the therapy of osteoporosis. The fragment, amylin-(1---8), stimulates osteoblast proliferation in vitro, although it is without effect on carbohydrate metabolism. The present study assessed the effects of daily administration of this peptide on sexually mature male mice for 4 wk. Amylin-(1---8) almost doubled histomorphometric indices of osteoblast activity but did not change measures of bone resorption. Trabecular bone volume increased by 36% as a result of increases in both trabecular number and trabecular thickness, and tibial cortical width increased by 8%. On three-point bending tests of bone strength, displacement to fracture was increased by amylin-(1---8), from 0.302 +/- 0.013 to 0.351 +/- 0. 017 mm (P = 0.02). In a separate experiment using dynamic histomorphometry with bone-seeking fluorochrome labels, amylin-(1---8) was administered by local injection over the calvariae of female mice. Amylin-(1---8) (40 nM) increased the double-labeled surface threefold. The effect was dose dependent from 0.4 to 40 nM and was greater than that of an equimolar dose of human parathyroid hormone-(1---34) [hPTH-(1---34)]. Mineral apposition rate was increased by 40 nM amylin-(1---8) but not by hPTH-(1---34). Amylin-(1---8) thus has significant anabolic effects in vivo, suggesting that this peptide or analogs of it should be further evaluated as potential therapies for osteoporosis.